Global model of low-temperature atmospheric-pressure He + H₂O plasmas

Abstract

A detailed global model of atmospheric-pressure He + H 2 O plasmas is presented in this paper. The model incorporates 46 species and 577 reactions. Based on simulation results obtained with this comprehensive model, the main species and reactions are identified, and simplified models capable of capturing the main physicochemical processes in He + H 2 O discharges are suggested. The accuracy of the simplified models is quantified and assessed for changes in water concentration, input power and electrode configuration. Simplified models can reduce the number of reactions by a factor of ∼10 while providing results that are within a factor of two of the detailed model. The simulation results indicate that Penning processes are the main ionization mechanism in this kind of discharge (1-3000 ppm of water), and water clusters of growing size are found to be the dominant charged species when the water concentration is above ∼100 ppm. Simulation results also predict a growing electronegative character of the discharge with increasing water concentration. The use of He + H 2 O discharges for the generation of reactive oxygen species of interest in biomedical applications and the green production of hydrogen peroxide are also discussed. Although it would be unrealistic to draw conclusions regarding the efficacy of these processes from a zero-dimensional global model, the results indicate the potential suitability of He + H 2 O plasmas for these two applications.

Time-resolved absolute OH density of a nanosecond pulsed discharge in atmospheric pressure He–H2O: absolute calibration, collisional quenching and the importance of charged species in OH production

“…So for the present conditions, the dominant primary ion will then be He + . These helium ions will be lost, apart from electronion recombination reactions, in the following charge transfer reactions [32]: …”

“…He metastables can create OH in Penning ionization reactions such as [32] . OH can also be produced (17) with a rate of 2.5 × 10 −46 m 6 s −1 [33].…”

“…It is suggested in the work of Stevefelt et al that this low density is potentially due to a collisional coupling to the radiating A 1 + u state. In addition, Liu et al studied He-H 2 O mixtures by a global model for different H 2 O concentrations, for discharges with an electron density of 10 17 -10 18 m −3 , and found a molecular metastable density which is consistently smaller compared to the atomic metastable density [32]. Finally, in microwave discharges at elevated temperatures (2500 K) it is also assumed that the dissociation of the dimer in collisions with a ground state helium atom is very fast, leading to a small molecular metastable density [35].…”

Chemistry in glow discharges of H2/O2mixtures: diagnostics and modelling

“…19 ). For HO 2 + , the reaction with H 2 to form H 3 + is nearly thermoneutral, with the forward (T 25 ) and backward (T 13 ) rate coefficients being comparable, and this equilibrium determines its concentration for H 2 -rich mixtures. When oxygen is the major component of the mixture, reaction T 24 becomes the main source of HO 2 + , while the destruction mechanism is still the same.…”

“…Atmospheric pressure discharges have been used in experimental and theoretical studies of H 2 + O 2 ignition [21], in the simulation of gas heating processes in H 2 + O 2 streamers [22], or in the modelling of production mechanisms of different neutral species [23,24]. An extensive global model for mixtures of He with small fractions (< 1%) of H 2 O has been elaborated by Liu et al [25], and the formation of OH radicals in plasma assisted combustion of H 2 / air mixtures has been studied experimentally and theoretically by Yin et al [26]. Low pressure plasmas of H 2 + O 2 , mostly in the mbar range, have been employed by various groups.…”